Generally, a semiconductor device has a substrate with a source region and a drain region separated by a channel region, and an insulating layer and a gate electrode sequentially stacked on the channel region. A three-terminal transistor uses the source region, the drain region, and the gate electrode as its terminals.
To form a transistor on a semiconductor substrate, a gate insulating layer is first formed on top of a substrate, and a conductive material such as a polysilicon layer is formed on top thereof.
Then, the gate insulating layer and the polysilicon layer are etched to form a gate electrode. Here, the corners of the gate electrode are rounded by the characteristics of the polysilicon.
Then, the semiconductor device is heat treated to form an oxide layer on the exposed surface of the gate electrode, and impurity ions are implanted to form a low density impurity region.
Next, a nitride layer is applied to the entire surface of the semiconductor device. Then, the nitride layer and the oxide layer are etched to form sidewalls on the gate electrode.
Once the sidewalls are formed, impurity ions are implanted into the semiconductor substrate to form a high density impurity region, completing the transistor.
However, in the prior art, the regions of a semiconductor substrate where the impurity ions are implanted, overlap with a lower region of the gate electrode. Accordingly, current may leak from the transistor. Consequently, the reliability of the semiconductor device decreases.